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P/V clearance measurement techniques

General engine tech -- Drag Racing to Circle Track

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hoodeng
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Re: P/V clearance measurement techniques

Post by hoodeng »

Class, your test is proof of rocker deflection, the o/p was looking for valve to piston.
As Bill pointed out, you will not feel a valve snicking a relief if the main load you are rotating against is full spring pressures.

My method is probably more at home in the mock up phase of a build, but then this is the time to be confirming and recording all clearances, not after a build.

Below are two cases, the bottom one was reported as a light ticking noise after a what i would have called a minor cam change, easy enough to cut the valve reliefs to suit but i binned the valves.
The top was a set of pistons supplied with both valve reliefs cut to the exhaust radius only, a shop sent them with cylinders to be bored, they did not notice the fault. If that engine cranked over with those pistons the damage would have been legendary, bent valves, broken guides, bent pushrods.

At the end of the day you can't beat documenting your work.
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donclark
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Re: P/V clearance measurement techniques

Post by donclark »

Jeff Lee wrote: Wed Nov 11, 2020 2:41 am
tenxal wrote: Tue Nov 10, 2020 7:24 pm If you have an LSM valve spring checker (everybody should have one...they are slick), part # PV100 is an anvil that lets you check piston-to-valve clearance with your race springs on. 8)
This is what I do also. I now have a roller cam and I use actual race springs which are 1,050 # open. The handle supplied is 2-piece so you can go short or long handle. Even long handle is a chore with this much pressure!
How to get your dial indicator on something steady with full valve train in place and valve moving up & down? Stick a single edge razor blade between the retainer and spring. Use razor blade as your dial indicator pickup point.
Got the LSM piece, worked like a charm. The razor blade was a great idea. Ended up measuring 0.105" intake and 0.125" exhaust clearance, so the intake pocket can be filled in on the new pistons quite a bit. Thanks for the help!
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Re: P/V clearance measurement techniques

Post by FC-Pilot »

Donclark, did you do measurements with the cam timing moved around? Knowing that I often play with cam timing on the dyno I like to verify the clearance at different cam settings so I am not stuck at one spot because of clearance limits. Just my thoughts.

Paul
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Re: P/V clearance measurement techniques

Post by donclark »

Yes, but I have run this cam and head combo before, so fairly sure where it will want to be. There is room to go 2 degrees either way. Thanks for the heads up.
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Re: P/V clearance measurement techniques

Post by FC-Pilot »

donclark wrote: Tue Nov 17, 2020 9:33 pm Yes, but I have run this cam and head combo before, so fairly sure where it will want to be. There is room to go 2 degrees either way. Thanks for the heads up.
Good deal. My setups have always been slightly different combos so I had a need to play around a little bit. It sure is nice to know your combo isn’t it.

Paul
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Re: P/V clearance measurement techniques

Post by tenxal »

donclark wrote: Tue Nov 17, 2020 7:38 pmGot the LSM piece, worked like a charm. The razor blade was a great idea. Ended up measuring 0.105" intake and 0.125" exhaust clearance, so the intake pocket can be filled in on the new pistons quite a bit. Thanks for the help!
Glad it worked out. Normally, we see about .025-.030 more V-P clearance with the race springs on versus a checking spring. About the only time the razor blade deal doesn't work is with the beehive/conical springs and the tiny retainers. Other than that, it's pretty slick. NHRA Tech officials have used this trick for many years when checking the lift on Stocker cams.

Best of luck with the project. :)
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Re: P/V clearance measurement techniques

Post by gmrocket »

If you have no piston to work with it’s a different story

Every time the piston manufacturer asks for free drop at over lap, they make the reliefs way to deep for me, even though I know exactly what it is with running springs and deflection loss. It’s still too deep

So I’ve resorted to giving them the relief depth I want . And I get what I ask for .

If you have no piston..this is what I do;

Cut a piece of stiff styrofoam, like SM blue the diameter of the bore..Fit it tight in the bore down far enough it won’t get contact from the valve...plasterscene over it that will cover the whole valve area and scrape it level with the deck.

With your head installed with gasket and running springs , roll it over

Then check your depression..don’t matter what your piston deck height will be, just add of subtract from the depression..
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